On the new commentaries from the A.D. volume seven DVD set it is officially revealed that they don't know what to carry on continuity wise from episode to episode!
I.E. hence why nobody knows wtf age Hayley is... they just make the shows and try the best to be on the same page... and they often fail.
Matt Weitzman spills the beans, basically stating they are more worried about writing jokes than paying attention to what continuity is going on, and the fans can like it or not.
For example: Jeff and Hayley move back in with the Smiths in "There Wil Be Bad Blood", yet several episodes later when the whole house gets evacuated in "Fartbreak Hotel" Jeff and Hayley aren't there.
Clearly the episodes ran (aired) out of the intended order.. and we're produced in an equally confusing way.
Listening to Weitzman talk on the commentary that he's so easily confused by all of it.. I'm surprised they kept anything straight.. b/c of the way the entire show gets made.
Good for him admitting the some-what painful truth, honestly.
I'll put Weitzman's exact quotes up here after I re-watch all the epiosdes with commentaries.